Method of stabilizing vinylidene chloride copolymer latices and resulting latex



United States Patent f NETHOD 0F STABILIZING VINYLIDENE CHLO- RIDE COPOLYMER LATICES AND RESULTING LATEX Dallas G. Grenley, Midland, Burke A. Beebe, Gladwiu, and Harold J. Townsend, Saginaw, Mich, assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Filed Dec. 30, 1959, Ser. No. 862,776

9 Claims. (Cl. 26017.4)

This invention relates to a method of producing mechanically stable latexes containing, as the disperse phase, copolymers of vinylidene chloride and certain ethylenically unsaturated compounds copolymerizable therewith. Particularly, it relates to the method of producing mechanically stable latexes possessing low foaming char acteristics and excellent wetting properties.

Copolymers of predominant amounts of vinylidene chloride with at least one monomer of the group defined by acrylonitrile and the lower alkyl esters of acrylic and methacrylic acids are known to possess excellent resistance to oils, greases, chemicals generally, and water. Thin films of these materials, display a very low rate of water vapor transmission. Because of this desirable combination of properties, the polymers in question find wide application in the field of protective coating. A particularly important application is in the packaging of foodstuffs, where low water vapor transmission is required. For such use the materials are generally employed as a coating on a paper or a cellophane base. The coating may be applied to the base by usual methods of depositing on the surface thereof a thin layer of either a solution of the copolymer in an organic solvent or of a latex of the copolymer and subsequently driving off the solvent of the solution or the aqueous portion of the latex. The solution coating method is relatively expensive in that it requires the use of solvents which in turn necessitates additional process steps for the recovery of the solvents. Furthermore, the use of solvents may present both fire and toxicological hazards. For these reasons, the latex coating method is generally preferred.

The latex coating method requires that the polymer latex possesses both mechanical stability, low surface tension, and low foaming characteristics. A variety of materials is available which are effective to increase the stability and the wetting ability of the latexes. Almost invariably, however, the agent used has the highly undesired effect of increasing the foamability of the latex. Foaming cannot ordinarily be tolerated in a coating operation inasmuch as it gives rise to pin holes in the protective coating.

It is, therefore, a principal object of the invention to provide latexes of the copolymers described which are broadly adaptable to the formation of a continuous film or protective coating. Additionally, it is an object of the invention to provide such latexes having good mechanical stability and low surface tension together with low foaming characteristics. Other and related objects will be evident from the following description.

According to the present invention, the above and related objects are attained by the addition to the latex of low emulsifier content of a small but effective amount of octakis-(Z-hydroxypropyl) sucrose monooleate as a surface tension lowering agent.

Latexes of low emulsifier content may advantageously be prepared by polymerization of the desired mixture of monomers in a redox catalyzed emulsion polymerization system in a manner as described in U.S. 2,462,422. Such latexes ordinarily possess adequate mechanical sta- 3,061 ,562 Patented Oct. 30, 1962 bility, but they are deficient in Wetting ability and are therefore not generally suited to coating applications. The addition of surfactants to these latexes in an :attempt to lower their surface tension almost invariably causes a marked decrease in mechanical stability. Frequently, this decrease in mechanical stability is of suflicient magnitude to cause coagulation. Furthermore, the addition of most surfactants normally has the added disadvantage of greatly increasing the foaming characteristics of the latex.

Latexes of this type to which octakis-(Z-hydroxypropyl) sucrose monooleate have been added possess'the requisite mechanical stability for use in the coating of paper by the usual methods of calendaring or by application with a doctor blade. They exhibit 'a minimum tendency to foam formation and, as a result, pin hole development in the coating film is negligible. Additionally, the surface tension of such latexes is sufliciently low that they readily wet the substrate to which they are applied.

The wetting and stabilizing agent itself may readily be prepared by the esterification of octakis-(Z-hydroxypropyl) sucrose as described in U.S. 2,908,681. Specifically, it may be made by the reaction of equimolar amounts of octakis-(Zhydroxypropyl) sucrose and methyl oleate in the presence of a small amount of sodium methoxide or similar alkaline esterification catalyst.

Inasmuch as the octakis-(Z-hydroxypropyl) sucrose is polyfunctional, it is possible for more than one of the hydroxypropyl groups in a given molecule to transesterify with the methyl oleate. Accordingly, it is to be expected that the desired monooleate will be contaminated with minor amounts of diester and possibly of the triester and higher esters. However, purification of the reaction product to isolate the desired monooleate in a substantially pure state is not necessary. 'lhe product -of the transesterification reaction described is effective to produce the desired stabilization without such purification.

The copclymers which constitute the disperse phase of the latexes to be stabilized by the method of the invention are materials particularly suited to use in food packaging by virtue of their high vinylidene chloride content. Vinylidene chloride may constitute from about 60 percent by weight of the polymerizable mixture to about 94 percent of this weight. The balance of the polymerizable mixture may be acrylonitrile or a lower alkyl ester of either acrylic or methacryl-ic acid in which the alkyl group may contain from one to eight carbon atoms. It may also be a mixture of acrylonitrile and such ester, the relative proportions of the components being a matter of choice dependent on the properties desired in the final film.

The octakis-(Z-hydroxypropyl) sucrose monooleate may generally be used in the proportion of from :about 0.2 percent to about 5 percent by weight of the polymer content of the latex. Preferably, there will be employed from about 0.5 percent to about 3 percent by weight of the weight of polymer. An amount of the monooleate from this preferred range will be found effective to provide the latex with excellent wetting properties without a concomitant and undesirable increase in foaming characteristics or decrease in mechanical stability. The monooleate may conveniently be added to the .la'texin the form of an aqueous solution containing about .25 percent: by weight of the material.

The invention is illustrated by the following examples, which are intended to be merely demonstrative and not limiting, and wherein all parts and percentages are on :a weight basis.

Example 1 In 100 parts of water there were dissolved 0.5 part of potassium persulfate, 0.3 part of sodium bisulfite and 0.025 part of an alkyl aryl sulfonate wetting and emulsifying agent. This solution was placed in a glass lined pressure vessel fitted with an agitator. A mixture of 85 parts of Vinylidene chloride, 10 parts of acrylonitrile and 5 parts of ethyl acrylate was introduced into the vessel and the free space of the vessel was flushed out with nitrogen. The contents were then agitated at 45 C. for a period of about two hours until a marked drop in the pressure in the vessel indicated completion of the reaction. The vessel was then cooled and vented and the latex removed.

The surface tension of the latex was determined and found to be 68 dynes per centimeter.

To 200 parts of the latex containing about 100 parts of polymer there was added sufficient ammonium hydroxide to bring the pH to a value of about 8.0. There were then added 3 parts of octakis-(Z-hydroxypropyl) sucrose monooleate as a solution in water of 25 percent concentration of the surfactant. The surface tension of the thus treated latex was determined and found to be 38 dynes per centimeter.

Example 2 The mechanical stability of the treated latex was measured by placing 200 cc. of the latex in a cylindrical container approximately 4 inches in diameter and subjecting it to the stirring action of a Hamilton Beach Beverage Mixer (Model 30) at a speed of approximately 18,000 revolutions per minute. This stirring was continued for one hour during which period only a negligible amount of foam was formed. The latex was then filtered through a fine mesh fabric. No coagulum was obtained.

Example 3 In 100 parts of water there were dissolved 0.5 part of potassium persulfate and 2.5 part of an alkyl aryl sulfonate emulsifying agent. To this solution, contained in the glass lined pressure vessel, there were added 100 parts of the monomeric mixture employed in Example 1. Emulsion polymerization was carried out as in Example 1 to obtain a latex containing about 50 percent of polymer.

Following the procedure of Example 2, the mechanical stability of the latex was determined. Complete coagulation occurred within a 5 minute period of stirring. Furthermore, foam was produced equal in volume to about 4 volumes of latex used.

Example 4 To a portion of the latex of Example 3 there were added 2.0 parts of alkyl aryl sulfonate to give a total emulsifier content of 4.5 parts per 100 parts of polymer in the latex. When subjected to high speed stirring in the mechanical stability test the thus stabilized latex coagulated completely in about 30 minutes. During the stirring there Was produced a volume of foam equal to approximately 6 times the original liquid volume.

Example 5 Following the procedure of Example 1 latexes were prepared using the aqueous phase described in that example and the following mixtures of monomers:

(a) Vinylidene chloride (90 parts), acrylonitrile (10 parts) (b) Vinylidene chloride (80 parts), methyl acrylate (10 parts), butyl acrylate (10 parts) Vinylidene chloride (85 parts), acrylonitrile (13 parts), octyl acrylate (2 parts) (d) Vinylidene chloride (70 parts), methyl acrylate (20 parts), ethyl acrylate 10 parts) (e) Vinylidene chloride (60 parts), methyl acrylate (30 parts), acrylonitrile (10 parts) The surface tension of each of these latexes was determined. In all cases values in excess of 65 dynes per centimeter were obtained.

Example 6 To each of the latexes of Example 5 there were added 3 parts of octakis-(2-hydroxypropyl) sucrose monooleate as a solution in water. In each case the addition of the surfactant brought about a lowering of the surface tension to a value of less than 40 dynes per centimeter.

Example 7 Each of the latexes of Example 6, containing 3 parts of octakis-(Z-hydroxypropyl) sucrose monooleate per 100 parts of polymeric product, were subjected to high speed stirring in the mechanical stability test for a period of a. full hour. Only negligible foam formation was observed. Filtration of the latexes through a fine mesh cloth indicated the complete absence of coagulum.

Example 8 A small amount of the untreated latex of Example 1 was poured on a smooth surface of polystyrene and spread out into a film by wiping with a doctor blade. The film broke immediately after being formed and the latex gathered into small droplets on the surface.

Example 9 To the latex of Example 8 there was added 0.2 part of octakis-(Z-hydroxypropyl) sucrose monooleate per 100 parts of polymer. This latex was wiped on the polystyrene surface as described above. A film formed, which broke in several places to reveal the polystyrene surface. However, gathering of the latex into small droplets did not occur. Drying gave a broken film.

Example 10 Example 8 was repeated employing the latex to which had been added 0.5 part of octakis-(Z-hydroxypropyl) sucrose monooleate. A non-broken wet film was obtained. Drying of this wet film gave a continuous sheet of the polymeric material.

What is claimed is:

1. A method for the stabilization of a synthetic resinous copolymer latex of low emulsifier content and having as the disperse phase thereof a copolymer of Vinylidene chloride and at least one member of the group consisting of acrylonitrile, lower alkyl esters of acrylic acid and lower alkyl esters of methacrylic acid in which the vinylidene chloride constitutes, in chemically combined form, from about 60 percent to about 94 percent by Weight of said copolymer, said method comprising admixing with said latex an amount of octakis-(2-hydroxypropyl)sucrose monooleate sufficient to reduce the surface tension of said latex to a value not exceeding 50 dynes per centimeter.

2. A method for the stabilization of a synthetic resinous copolymer latex of low emulsifier content and having as the disperse phase thereof a copolymer of Vinylidene chloride and at least one member of the group consisting of acrylonitrile, lower alkyl esters of acrylic acid and lower alkyl esters of methacrylic acid in which the vinylidene chloride constitutes, in chemically combined form, from about 60 percent to about 94 percent by weight of said copolymer, said method comprising admixing with the said latex from about 0.2 part to about 5.0 parts by weight, on the basis of 100 parts by weight of the copolymer contained in the latex, of octakis-(Z-hydroxypropyl)- sucrose monooleate.

3. The method according to claim 2, wherein the chemically combined Vinylidene chloride constituted from about percent to about 90 percent by weight of the copolymer.

4. The method according to claim 2, wherein the copolymer contains, in chemically combined form, about percent by weight of vinylidene chloride, about 10 percent by weight of acrylonitrile and about 5 percent by Weight of a lower alkyl ester of acrylic acid.

5. The method according to claim 2, wherein the polymerizable mixture comprises a mixture of about 85 percent by weight of vinylidene chloride, about 10 percent by weight of acrylonitrile and about 5 percent by Weight of butyl acrylate.

6. The method according to claim 2, wherein the copolymer contains, in chemically combined form, about 85 percent by weight of vinylidene chloride, about 10 percent by weight of acrylonitrile and about 5 percent by weight of octyl acrylate.

7. The method according to claim 2, wherein the polymerizable mixture comprises a mixture of about 90 percent by weight of vinylidene chloride and about 10 percent by weight of acrylonitrile.

8. The method according to claim 2, wherein the polymerizable mixture comprises a mixture of about 90 percent by weight of vinylidene chloride and about 10 percent by weight of methyl acrylate.

9. A latex prepared according to the method of claim 2.

References Cited in the file of this patent UNITED STATES PATENTS 2,462,422 Plambeck Feb. 22, 1949 2,908,681 Anderson et a1. Oct. 13, 1959 2,941,971 Thompson et a1 June 21, 1960 

1. A METHOD FOR THEW STABILIZATION OF A SYNTHETIC RESINOUS COPOLYMER LATEX OF LOW EMULSIFIER CONTENT AND HAVING AS THE DISPERSE PHASE THEREOF A COPOLYMER OF VINYLIDENE CHLORIDE AND AT LEAST ONE MEMBER OF THE GROUP CONSISTING OF ACRYLONITRILE, LOWER ALKYL ESTERS OF ACRYLIC ACID AND LOWER ALKYL ESTERS OF METHACRYLIC ACID IN WHICH THE VINYLIDENE CHLORIDE CONSTITUTES, IN CHEMICALLY COMBINED FORM, FROM ABOUT 60 PERCENT TO ABOUT 94 PERCENT BY WEIGHT OF SAID COPOLYMER, SAID METHOD COMPRISING ADMIXING WITH SAID LATEX AN AMOUNT OF OCTAKIS-(2-HYDROXYPROPYL)SUCROSE MONOOLEATE SUFFICIENT TO REDUCE THE SURFACE TENSION OF SAID LATEX TO A VALUE NOT EXCEEDING 50 DYNES PER CENTIMETER. 